$σ_{n} = σ_{1} - σ_{2} 2 + σ_{1} + σ_{2} 2 \cos 2 \theta + τ \sin 2 \theta$

$σ_{n} = σ_{1} + σ_{2} 2 - σ_{1} - σ_{2} 2 \cos 2 \theta - τ \sin 2 \theta$

$σ_{n} = σ_{1} + σ_{2} 2 + σ_{1} - σ_{2} 2 \cos 2 \theta - τ \sin 2 \theta$

$σ_{n} = σ_{1} + σ_{2} 2 + σ_{1} - σ_{2} 2 \cos 2 \theta + τ \sin 2 \theta$